The present invention relates to a method and apparatus for processing video signals, especially for enhancing visual image quality with correction of or adjustments to gradation and chrominance components of input video signals.
Video signals are processed with gamma characteristics before broadcast to display apparatus having a cathode ray tube (CRT). CRT display apparatus process these video signals with reverse gamma characteristics to display images at linear gradation.
Different from CRT display apparatus, plasma display panels (PDP) and liquid crystal display (LCD) apparatus, both types exhibiting the linear gradation characteristics, apply gamma correction to video signals already processed with gamma characteristics to process the signals with reverse gamma characteristics for displaying images at linear gradation.
Not only achieving linear gradation, gamma correction is applied to video signals to make adjustments to gamma curves constituted by input luminance signals Y-IN and output luminance signals Y-OUT in image-quality adjustments.
As prior art, for example, Japanese Unexamined Patent Publication No. 2006-033469 discloses a gradation correction apparatus including a gradation corrector having a gamma corrector that corrects gradation of luminance signals, a luminance-level histogram detector to detect the luminance-level histogram of the luminance signals, an edge-level histogram detector to detect the edge-level histogram of the luminance signals, and a gradation-correction characteristics setter to generate gradation correction characteristics based on detection results of the detectors and set the gradation correction characteristics in the gradation corrector.
The known gradation correction apparatus could, however, suffer improper gradation correction based on gradation distribution not necessarily always appropriate due to generation of a gradation correction curve with a steep gamma ramp at highly distributed luminance levels and a gentle gamma ramp at not highly distributed luminance levels in the luminance-level histogram with no consideration of image spatial information.
Adjustments to the gamma ramp, for example, a steeper gamma ramp for flat gradation (with almost no change in gradation) at a dark zone that occupies a relatively large zone of an image could cause a bigger gradation difference to the dark zone to emphasize false edge or noise components at the dark zone. It also could emphasize black floating at the dark zone to lower contrast, which leads to lowered image quality.
Moreover, a steeper gamma ramp for a high luminance-level section (such as, the sky), with almost no change in gradation, that also occupies a large zone of an image could cause an object (a building, human being, etc.) at an intermediate luminance-level section to suffer a gentler gamma ramp, resulting in lower contrast and dark impression to viewers.